Motorized control systems are in widespread use in advancing objects between one or more positions and where it is important to control cut-off of the motor drive immediately upon reaching a selected position or upon sensing an obstruction so that the motor or drive system is not damaged as a result of permitting the motor to run any length of time after the position or obstruction is reached. This problem is typified by the operation of window shades or blinds wherein the blind is normally intended to advance between upper and lower open and closed positions but often may encounter unexpected interference or obstructions in its path of travel. Unless the motive source of power can be stopped immediately, damage may be inflicted on one or more of the blinds, drive system and the power source.
Different approaches have been taken to solving the aforementioned problem, such as, by counting the number of revolutions between the end limits of travel of the blind, the use of limit switches at opposite end limits as well as the use of magnetic and piezoelectric motion sensors. Representative patents in this area which utilize the approaches discussed are U.S. Letters Patent Nos. 3,330,922 to R. E. Rowe, 3,559,024 to H. Marder, 4,319,236 to P. H. Brace et al, 4,665,965 to H. Pasquier et al, 4,735,251 to E. Tsuchida et al, 4,758,824 to P. Young and 4,856,574 to K. Minami et al. Nonetheless, none of these has suggested solving the problem by sensing the change in rate of signals or pulses generated in direct correlation with the rate of travel of the blind whereby the motive power source can be switched off immediately to avoid possible damage at any point along the path of travel. In sensing pulse rate changes, the system can achieve simplicity yet versatility in simultaneously controlling one or more moving objects, such as, window blinds in opening and closing over wide ranges in speeds and distances.